Relationship of Pelvic Positional Change with Leg Length and Offset Measurement in Experimental Total Hip Arthroplasty

OBJECTIVE: During total hip arthroplasty (THA), both pelvic and femur positions affect leg length (LL) and offset (OS) measurements because LL and OS calipers depend on the fixed reference points on the pelvis and femur, respectively. However, LL and OS measurement errors because of pelvic positional changes have not been described. This study aimed to clarify the effects of pelvic positional changes on LL and OS measurements in relation to the pelvic reference using a THA simulator. METHODS: We developed an experimental THA simulator using Sawbones models of the hemipelvis and femur that facilitated modification of the obliquity, tilt, and rotation of the pelvis. Using an LL and OS caliper, LL and OS measurement errors due to pelvic positional changes were determined with the femoral position fixed. Measurements were performed from two pelvic reference positions: the iliac tubercle (P1) and the top of the iliac crest intersecting the line of the femoral long axis (P2). RESULTS: Concerning pelvic obliquity, the total error of LL was 25.0 mm in P1 and 26.5 mm in P2, while the total error of OS was 13.0 mm in P1 and 10.9 mm in P2. For pelvic tilt, the total error of LL was 9.0 mm in P1 and 3.8 mm in P2, while the total error of OS was 0.5 mm in P1 and 1.0 mm in P2. Regarding pelvic rotation, the total error of LL was 13.8 mm in P1 and 3.2 mm in P2, while the total error of OS was 3.8 mm in P1 and 4.0 mm in P2. CONCLUSIONS: Pelvic positional changes alter LL and OS measurements. The acceptable range (error <2 mm) on LL and OS measurement errors of pelvic obliquity was only 2°, regardless of the pelvic reference position. The pelvic reference position should be at the top of the iliac crest intersecting the line of the long axis of the femur because of a small LL measurement error with pelvic tilt and rotation.